Telephone transmitters



1960 A. c. BEADLE TELEPHONE TRANSMITTERS 7 Filed 001;. 28, 1957 InventorA. C. Beadle Atlorne y TELEPHONE TRANSMITTERS Anthony Crisp Beadle,London, England, assignor to International Standard ElectricCorporation, New York, N.Y., a corporation of Delaware Filed Oct. 28,1957, Ser. No. 692,806

Claims priority, application Great Britain Nov. 9, 1956 8 Claims. (Cl.179-122) This invention relates to telephone transmitters of the carbongranules type.

According to the invention, there is provided a telephone transmittercomprising a casing, a carbon granules chamber within said casing, and adiaphragm supported directly or indirectly upon the said casing but freeto vibrate axially, the said diaphragm comprising a central electrodeportion co-operating with the said carbon chamber so as to act upon agranular mass of carbon contained therein, and in which the said carbonchamber contains two concentric fixed electrodes making contact with thesaid granular mass, the arrangement of the said fixed electrodes and thevolume of the granular mass provided being such that a substantiallyconstant resistance between the said fixed electrodes is maintained inall positions of the said transmitter, subject only to variationsarising from acoustically-generated axial excursions of the saiddiaphragm acting upon the said granular mass.

The principal object of the invention is to provide a high resistancecarbon transmitter of the series type, that is to say, one in which thetalking resistance is developed in a series path between two fixedelectrodes in the carbon chamber, and capable of beingvaried by theexcursions of the diaphragm. This path may, and in general does, includea conductive surface which may be a portion of the diaphragm orwhich'maybe a separate electrode driven by the diaphragm. The-resistance thereby developed is approximately four times that of theconventional transmitter utilising the single path between a backelectrode and the front (diaphragm) electrode. Thus the talkingresistance of the transmitter when new may be of the order 300400 ohmsas opposed to the value of around 80 ohms for the more usual type.

The particular choice of transmitter resistance is mainly determined bythe operating conditions of the DC. feeding bridge in the telephoneexchange, it being a necessary condition that the feeding current shallnot fall below the minimum required for operation of the supervisory orsignalling relay when the exchange loop is closed. Since the electricaloutput of a carbon transmitter is approximately proportional to the DC.power fed to it, it follows that exchange systems with low feedingcurrents will require high resistance transmitters whereas systems whichneed higher feeding currents must use relatively low resistancetransmitters.

Series-type transmitters hitherto have consisted, in the main, of anasymmetric arrangement of two or more side-by-side carbon chambersbridged by a fiat conductive diaphragm (electrode), frequently ofcarbon, and having the following limitations:

(i) Poor positional stability of resistance due to the use of a fiatfront electrode;

(ii) The side-by-side arrangement of the fixed back electrodes givesaxial asymmetry so that the transmitter capsule must be prevented fromrotation;

(iii) The use of moulded carbon electrodes is expen- 2,960,579 PatentedNov. 15, 1960 sive. Moreover, the use of felt inside the capsule, as insome of theproposed designs, is objectionable.

The presentv invention seeks to avoid these limitations by novelfeatures of construction, and the invention will now be described withreference to the single figure of the accompanying drawing, illustratinga preferred cmbodiment.

The main frame of the transmitter comprises two pressings 1 and 3rigidly connected by a chamber ring 2, the latter being manufactured ofdie cast metal, such as aluminum alloy insulated'on its inner surface,or moulded from plastic material. The fastening together may be achievedby punching or spinning over of lugs on the chamber ring (as shown inthe drawing) or by screws or rivets passing into or through the chamberring. The same fastenings secure one or more fabric washers 4 betweenthe partitions 3 and the chamber ring. These washers perform the dualfunction of closing the top end of the carbon chamber and providing adamping resistance across the holes 16. The acoustic mass of these holestogether with the volumes on the two sides of the partition between thediaphragm 7 and back cover forms the acoustic equaliser for thetransmitter, in known manner. The diaphragm 7 is protected and clampedat its outer edge by a front cover 5, which also secures the twopressings 1. and 3 at their outer edges.

The equalising and clamping arrangement just described may be achievedalternatively by a damping arrangement, in which the diaphragm issupported on tissue washers, having different damping effects atdifierent frequencies. This is an indirect method of damping.

An important feature of the transmitter lies in the shape andconstruction of the carbon chamber, which contains two fixed electrodesand a moving electrode 8, fixed to the diaphragm 7 so that it movesaxially with the diaphragm in response to the applied sound pressure.The two fixed electrodes consist of a central electrode 9 and aconcentric ring electrode 10 separated by an insulating bushing 11. Thebushing 11 protrudes well into the carbon chamber and supports thecentral electrode 9 which is shaped like a round-headed bolt, so thatthe head of the electrode rests in a cylindrical hollow in the bushing11. The ring electrode 10 is stepped, as shown, the head of the bushingresting on its upper step, while the lower step is secured between thechamber ring and the back of the frame between an insulating disc 12 andan insulating washer 15. Contact is made to the ring electrode 10 by theterminal 13, insulated from the framework of pressing 1 underneath bywasher 17, while the terminal end'of the central electrode is similarlyinsulated from the pressing l by washer 14. Additional insulation withinthe carbon chamber may be applied, as, for example, by enamel, to thecylindrical sides of both the front electrodes (where shown at 8) andthe ring electrode (where shown at 10).

The carbon chamber thus formed is substantially filled with granularcarbon, but not packed tightly, to allow for mobility of the carbonunder all conditions of temperature and usage.

With this construction, the insulating bushing 11 partially obstructsthe direct path for current between the electrodes 9 and 1t), and undernormal operating conditions, the main current path through the carbonmass traverses the carbon twice, from one fixed electrode, to the movingelectrode and back to the second electrode, and is thus subject to themodulating eifect of the acoustic excursions of the diaphragm. The shapeof the insulating barrier 11 and the stepping back of the ring electroderelative to the central electrode ensure that there is no low resistancetrack through the carbon granules, directly between the two fixedelectrodes, which might cause a loss of efiiciency. Small radial grooves(not shown) may be cut in the insulating barrier to increase themobility of the granular carbon.

A further feature of the carbon chamber construction is the relativepositioning of the electrodes and the bounding surfaces of the chamberto give a substantially constant resistance to the transmitterregardless of axial or diametric rotation. The former is achieved byaxial symmetry as is normal in two electrode transmitters, but which isbelieved to be novel for series type transmitters. The latter isachieved by positioning the active current paths efiectively near thecentre of the carbon chamber so that a useful working head of carbon ismaintained regardless of the plane of the transmitter; it will be notedthat in the face up position the working head of carbon on the centralelectrode is small but on the outer electrode large, whereas in the facedown position the reverse is true so that the effect of the two paths inseries is to maintain substantially constant re sistance.

Notwithstanding the above application of this trans mitter for use as ahigh resistance transmitter, satisfac tory operation may also beobtained by connecting the two current paths in parallel and adding aframe terminal with a conducting path to the moving electrode. Underthese conditions the resistance of the transmitter would be reduced toone quarter and it would operate satisfactorily at higher feed currents.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

What I claim is:

1. A telephone transmitter comprising a casing, a granular carbonchamber Within said casing, a pair of fixed concentric electrodes withinsaid chamber, an insulating member disposed intermediate said fixedelectrodes and partially shielding the inner electrode of said pair fromthe outer electrode, a diaphragm mounted on said casing and having acentral electrode portion coopcrating with said inner electrode, saidcentral electrode portion and a portion of said insulating memberdefining a relatively narrow aperture wherein a substantially constantresistance between said fixed electrodes is maintained in all positionsof said transmitter.

2. A telephone transmitter as claimed in claim 1, wherein saidinsulating member comprises a hollow cylinder having a truncatedcone-shaped portion at one end, the base of said cone-shaped portionhaving a wider diameter than the balance of said cylinder, a bore in theupper part of said cone portion having a diameter greater than thehollow in said cylinder.

3. A telephone transmitter as claimed in claim 2, wherein one of saidfixed electrodes comprises a cylinder having a dome-shaped portion atone end, the widest diameter of said dome-shaped portion being greaterthan the diameter of the balance of said fixed electrode, said fixedelectrode positioned within the hollow of said insulating member withits dome-shaped portion within said bore.

4. A telephone transmitter as claimed in claim 3, wherein the other ofsaid fixed electrodes comprises an inverted cup-shaped element having anoutwardly extending flange portion and a central aperture, the diameterof said aperture being substantially equal to the outside diameter ofsaid hollow cylinder, said insulating member extending through saidaperture, the bottom side f the base of said cone-shaped portion of saidinsulating member resting on the bottom of said inverted cup-shapedelement.

5. A transmitter as claimed in claim 1, wherein the inner electrode ofsaid pair of electrodes has its surface closer to said central electrodeof said diaphragm substantially hemispherical in shape.

6. A transmitter as claimed in claim 1 and in which the said carbonchamber comprises a pair of frame members mounted within said casing, arigid cylindrical ring supported between said frame members, one of saidframe members forming a backing plate and the other providing a seatingfor the said diaphragm, said insulating member seated centrally of thesaid chamber on said backing plate so as to protrude into the saidchamber, said inner electrode mounted centrally of said insulatingmember giving access at its inner end to the carbon mass in the saidcarbon chamber and at its outer end to a connecting terminal, said outerelectrode mounted concentrically to said insulating member provided witha connecting terminal, said chamber being closed by said diaphragm andfilled to a predetermined extent with said carbon.

7. A transmitter as claimed in claim 1, wherein the inner of the saidtwo fixed electrodes is annular and projects into the said carbonchamber.

8.A transmitter as claimed in claim 1, wherein the central electrodeportion of said diaphragm comprises a cylindrical portion concentricwith the said fixed electrodes, the working surface within the saidelectrode portion being re-entrant and domed outwards from the saidcarbon chamber.

References Cited in the file of this patent FOREIGN PATENTS 153,888Australia -i Oct. 30, 1953

